.box {
  width: 100px;
  height: 100px;
  background: red;
}

* {
  margin: 0;
  padding: 0;
}

.app {
  width: 100vw;
  height: 100vh;
  display: flex;
  flex-direction: column;
  header, footer{
    display: flex;
    justify-content: space-around;
    align-items: center;
    background: skyblue;
    height: 36px;
  }

  main {
    flex: 1;
    height: 100%;
    overflow-y: scroll;
  }
}